Zinc dialkyldithiophosphate additives are used to control wear and inhibit oxidation in almost all engine oils as well as many other types of lubricant. They limit wear primarily by forming a thick,
protective, phosphate glass-based tribofilm on rubbing surfaces. This film formation can occur at low temperatures and is relatively indifferent to the chemical nature of the substrate. There has been considerable debate as to what drives ZDDP tribofilm formation,
why it occurs only on surfaces that experience sliding and whether
film formation is controlled primarily by temperature, pressure, triboemission or some other factor.

This paper describes a novel approach to the problem by studying the formation of ZDDP films
in full film EHD conditions from two lubricants having very different EHD friction properties. This shows that ZDDP film formation does not require solid-solid rubbing contact but is driven simply by applied shear stress, in accord with a stress-promoted thermal activation model. The shear stress present in a high pressure contact can reduce the thermal activation energy for ZDDP by
at least half, greatly increasing the reaction rate. This mechanism
explains the origins of many practically important features of ZDDP films; their topography, their thickness and the conditions under which they form.

The insights that this study provides should prove valuable both in
optimising ZDDP structure and in modelling ZDDP antiwear behaviour. The findings also highlight the importance of
mechanochemistry to the behaviour of lubricant additives in general.